The present invention relates to an electric power assisted steering system.
Electric power assisted steering systems are well known in the art. Electric power assisted steering systems utilizing a rack and pinion steering gear provide power assistance by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to the rack bar of the rack and pinion steering gear.
In known electric power assisted steering systems, a controller controls the electric motor in responsive to (i) torsion across a torsion bar that is interposed between the steering wheel of the vehicle and a pinion gear and (ii) vehicle speed. Power assistance from the electric motor causes the rack bar of the steering gear to move linearly relative to the housing of the steering gear. Movement of the rack bar relative to the steering gear housing causes the steerable wheels of the vehicle to turn.
It is well known that friction resists the movement of the rack bar relative to the housing of the steering gear. The friction typically results from a sliding movement of the rack bar over parts of the steering gear, such as a bushing, a yoke assembly, and seals.
The present invention is an electric power assisted steering system for a vehicle. The electric power assisted steering system comprises a steering gear having a rack bar and a housing. The rack bar is movable linearly relative to the housing for turning steerable wheels of the vehicle. Movement of the rack bar relative to the housing is resisted by friction. An electric motor is connected with the rack bar of the steering gear. Energization of the electric motor causes linear movement of the rack bar relative to the steering gear housing. The electric power assisted steering system further includes a vehicle speed sensor for monitoring the speed of the vehicle and generating a signal indicative of the vehicle speed, a torsion sensor for monitoring the torsion across a torsion bar of the steering gear of the vehicle and generating a signal indicative of the torsion, and a controller for receiving the vehicle speed and torsion signals and, in response to the vehicle speed and torsion signals, for generating a motor control signal for controlling the electric motor. The controller stores data indicative of the friction resisting movement of the rack bar relative to the housing of the steering gear. The controller modifies the motor control signal to compensate for the friction.
The present invention also is a method for controlling an electric power assisted steering system for a vehicle. The electric power steering system includes a steering gear, an electric motor, and a controller for controlling the electric motor. The steering gear has a rack bar that is movable relative to a steering gear housing. Energization of the electric motor causes linear movement of the rack bar relative to the housing of the steering gear. Movement of the rack bar relative to the housing is resisted by friction. The method comprises the steps of (i) providing a vehicle speed signal indicative of a speed of the vehicle; (ii) providing a torsion signal indicative torsion across a torsion bar of the steering gear of the vehicle; (iii) generating a motor control signal in response to the torsion and vehicle speed signals; (iv) determining a value for the friction that resists movement of the rack bar relative to the housing of the steering gear; and (v) modifying the motor control signal to compensate for the friction.